GdkSpan is a structure holding a spanline. A spanline is a horizontal line that
is one pixel wide. It is mainly used when rasterizing other graphics primitives.
It can be intersected to regions by using gdk_region_spans_intersect_foreach().

gdk_rectangle_union ()

Calculates the union of two rectangles.
The union of rectangles src1 and src2 is the smallest rectangle which
includes both src1 and src2 within it.
It is allowed for dest to be the same as either src1 or src2.